This invention relates to an improved aluminum alloy product for use in making heat exchanger fins, and more particularly to a fin stock material having both a high strength and a high thermal conductivity.
Aluminum alloys have long been used in the production of heat exchanger fins, e.g. for automotive radiators, condensers, evaporators etc. Traditional radiator fin alloys are designed to give a high strength after brazing, a good brazability and a good sag resistance during brazing. Alloys used for this purpose usually contain a high level of manganese. An example is the aluminum alloy AA3003. Such alloys provide a good brazing performance; however, the thermal conductivity is relatively low. This low thermal conductivity was not a serious problem in the past because the major thermal barrier for fin stock was the fin-to-air heat transfer. Recently, there has been a demand for radiators having increased heat transfer efficiency. These new generation radiators require a new fin material which has a high strength as well as a high thermal conductivity.
The new fin material properties demanded by the automotive heat exchanger industry includes a high ultimate strength (UTS) after brazing, a high brazing temperature and a high conductivity for fin material having a thickness of no more than about 0.1 mm.
Morris et al., U.S. Pat. No. 3,989,548 describes an aluminum alloy containing Fe, Si, Mn and Zn. These alloys preferably are high in Mn which would result in adequate strength but poor conductivity. The alloys are not described as being useful for fin stock.
In Morris et al., British Patent 1,524,355 there are described dispersion-strengthened aluminum alloy products of the Alxe2x80x94Fe type which typically contain Fe, Si, Mn and Cu. The Cu is present in amounts up to 0.3% and this has a negative effect on conductivity and causes pitting corrosion, both of which would be particularly detrimental to performance of very thin fins.
An alloy that is said to be useful for heat exchange fin stock is described in Morris et al, U.S. Pat. No. 4,126,487. That aluminum alloy contains Fe, Si, Mn and Zn. It preferably also contains some Cu and Mg for added strength. As with GB 1,524,355, the Cu may be present in amounts up to 0.3%, which would be detrimental to the performance of very thin fins.
It is an object of the present invention to produce a new aluminum alloy fin stock which has both a high strength and a high thermal conductivity.
The present invention relates to a novel fin stock material that is suitable for manufacturing brazed heat exchangers using thinner fins than previously possible. This is achieved while retaining adequate strength and conductivity in the fins to permit their use in heat exchangers.
The above combination of characteristics has surprisingly been obtained according to the present invention by balancing three somewhat contradictory properties in the material, namely strength (UTS) after brazing, electrical/thermal conductivity after brazing and brazing temperature (melting point of fin material during a brazing operation).
One problem in developing this type of alloy is meeting the conductivity requirements. Thus, if the conductivity is improved by modifying a traditional alloy composition, for example by reducing the Mn content of alloy AA3003, then the strength of the alloy becomes too low. It was found that the desired balance of characteristics could be obtained by starting with a material in which there was a certain amount of particle based strengthening, which does not normally have a negative effect on conductivity. Elements were then added that contribute to solution strengthening in a carefully selected manner so as to raise the strength without lowering the conductivity or melting temperature to an extent that would make the material unusable. A microstructure was developed which provides an optimum combination of particle hardening and solid solution strengthening by introducing a high volume fraction of uniformly distributed fine intermetallic particles. To maximize the effect of particle and solution strengthening at a given composition, so that the desired properties are achieved, a high cooling rate strip casting procedure was required, but not so high as to retain excess conductivity destroying elements in solid solution.
The aluminum alloy of the invention has the composition (all percentages by weight):
The strip product formed from this alloy according to the present invention has a strength (UTS) after brazing greater than about 127 MPa, preferably greater than about 130 MPa, a conductivity after brazing greater than 49.8% IACS, preferably greater than 50.0% IACS and a brazing temperature greater than 595xc2x0 C., preferably greater than 600xc2x0 C.
These strip properties are measured under simulated brazed conditions as follows.
The UTS after brazing is measured according to the following procedure which simulates the brazing conditions. The processed fin stock in its final as rolled thickness (e.g. after rolling to 0.06 mm in thickness) is placed in a furnace preheated to 570xc2x0 C. then heated to 600xc2x0 C. in approximately 12 minutes, held (soaked) at 600xc2x0 C. for 3 minutes, cooled to 400xc2x0 C. at 50xc2x0 C./min then air cooled to room temperature. The tensile test is then performed on this material.
The conductivity after brazing is measured as electrical conductivity on a sample processed as for the UTS test which simulates the brazing conditions, using conductivity tests as described in JIS-H0505.